1. Field of the Invention
This invention is generally directed to chemical and biochemical quantitative analysis, and more specifically concerns an optical fiber sensor for measuring pH in a fluid or gaseous mixture.
2. Description of Related Art
In modern medicine, measurement of acidity (pH) in the blood has become an important factor in the determination of the respiratory status of a patient. Although electrodes have been developed which are capable of measuring pH in fluids, they are of limited use in measurement of in vivo blood pH levels. Optical sensors for taking intravascular measurements of acidity and other blood analytes such as oxygen and carbon dioxide show promise for in vivo measurement of blood pH. Such optical pH sensors typically include a fluorescent indicator dye, such as fluorescein or hydroxypyrenetrisulfonic acid (HPTS), placed over the tip of an optical fiber and a membrane cover over the dye which is permeable to the hydronium ions to be measured. The dye fluoresces when exposed to a certain wavelength of light conducted to it by the optical fiber. In practice, a pH sensor is fabricated by immobilizing a pH sensitive dye into a matrix attached to the distal end of the fiber. The dye is typically capable of existing in two forms, an anionic or base form, and a protonated or acid form. The two forms are each excited by a different frequency, but fluoresce at the same frequency, with the output responsive to excitation at the appropriate different frequencies being proportional to the pH of the sample to which the sensor is exposed. In this manner, measurement of the intensity of fluorescence of the indicator dye can be related to pH.
Optical absorbance indicator dyes, such as phenol red, have also been utilized in optical pH sensors. In this type of pH sensor, green and red light are emitted from one end of the optical fiber into the dye material, passing through the dye to be reflected back into another optical fiber. The green light is absorbed by the base form of the indicator, and the red light is not absorbed by the indicator, so that it may be used as an optical reference. The ratio of green to red light can thus be related to pH.
One approach to construction of optical fiber sensors involves the attachment of a dye filled porous glass to the tip of the optical fiber, such as by an adhesive. Another approach has involved the application of sensing material directly to the tip of the optical fiber. Another approach has involved the attachment of a sleeve which contains the dye indicator sensing material immobilized in a hydrophilic polymeric matrix by entrapment or by ionic interactions over the tip of the optical fiber. However, such sensors allow the indicator dye to leach out over extended time periods. Leaching of the indicator dye results in increasingly inaccurate blood pH measurements. Other covalently bonded sensors known in the art have either not been capable of attachment to the end of the optical fiber, or have been merely cast over the tip of the fiber without being crosslinked or covalently attached to the fiber.
There remains a need for a fiber optic pH sensor which provides covalent linkages between the dye and matrix, and between the matrix and the optical fiber, to prevent leaching of the indicator material during periods of extended use of the sensor in measuring blood pH intravascularly. It would also be desirable to allow for control of the concentration of dye in the final sensor matrix, and to allow for uniform application of the sensor matrix over a wide range of sensor thicknesses.